With operations handling massive quantities of bulk materials every day, the dust generated can become a serious health hazard to mining personnel and the surrounding community. Corin Holmes, Operations Manager at Jenike and Johanson, explains how operations can reduce these risks.
Trends in material handling over the last decade have exacerbated the problem of dust generation. Much of the materials handled is transported at much higher speeds than in the past, increasing the potential for entrained dust. Improper design of materials handling equipment can further exacerbate the issues associated with dust contamination with hazards affecting operations, health of workers and surrounding communities, and in some cases even cause fire or explosions.
Dust generation requires three elements analogous to a framework such as that of a ‘fire triangle’ (see Figure 1):
• Presence of fine particles capable of being entrained in air present in either the feed material or generated during handling (such as attrition/degradation of particles).
• Amenability of the fine particles to be entrained (low particle to particle cohesion).
• Elutriation – a process of separating bulk solid particles based on size, shape, and density.
The primary mechanism of dust generation is the dispersion of fine material particles into turbulent air streams that develop with a falling material stream (see Figure 2). Some of the kinetic energy of the falling material stream is transferred from the material to air in the stream and, to a lesser degree, the surrounding air that enters the stream at the boundaries. It is quite common to see of dust generation during bulk solids transfer.
At the boundary between the material stream and surrounding air, some of the fine particles can be stripped from the material stream due to frictional drag and then carried away (this mechanism is depicted in Figure 3).
Turbulence is also generated when the air-entrained particle stream strikes a surface/object reducing the stream velocity. As a result, the air stream and fine particles that are carried along with it, are forced to escape. This mechanism can form billowing clouds of dust particles that can travel hundreds of meters from the source.
DUST CONTROL METHODS
DUST COLLECTION
The primary method used to control dust is by aspiration (the process of removing dust from the air) while the bulk solid is being handled. This is usually accomplished by the dust collection system which takes the dusty air and conveys it to a filtration system for dust separation and collection. Once the dust is collected the particulates are often recirculated back into the material handling system. There are four key components of a dust collection system: the filter, duct work, pickup hoods, and the air mover. Many vendors provide excellent components that do the job; however, the key is to get the components to work as a cohesive unit to provide effective dust collection.
An example dust collection system is shown in Figure 4. Dust generated at a point source is pulled into pickup hoods and the air flow through the hoods is be controlled by either blast gates (common) or by duct sizing (less common, but more effective). The duct work (called branching and trunk lines) is routed through the facility, often changing directions multiple times, and likely increasing in diameter along the length. The trunk of the ducting terminates at the filter/collector where the particles are separated from the air stream. The filter operates by either an inertial or a physical barrier (cloth ‘sock’ or fabric, sintered metal) as a means of retaining the particles while the air leaves the collector.
The heart of the dust collection system is the air mover/filter combination. The symbiotic relationship between the two components is important. If the filter becomes clogged with dust, the fan performance will likely be reduced causing major system problems.
When it comes to dust collection design one should consider a ‘system design’ engineering approach versus a “component selection” method. There are several tips to ensure a well operated system [2]:
• Tip #1 – Ensure sufficient conveying velocity. Sufficient conveying velocity is required to pick up the dust from the pickup hoods and transport the material to the filter/collector for separation from the air stream. If you can’t effectively convey the dust, then you will not collect the dust, other than in the horizontal duct work.
• Tip #2 – implement balance-by-design. The balance by design method is the preferred method to ensure adequate conveying velocity in each portion of the system. With balance by design, the duct size and layout are engineered to balance the branches, pickups, and trunk layouts based on static pressure loss through each section. If each branch has a similar static pressure loss, then the air flow, and thus velocity, through each branch will be equal. This avoids the need to use blast gates or orifice plates to manually adjust air flow per branch.
• Tip #3 – Select the right air mover. The selection of an appropriate air mover is straight forward once the system resistance is determined [3]. Fans and positive displacement (PD) rotary blowers are typically used to provide the suction force for the dust collection system. It is important when designing the dust collection system that anticipation of maximum solids loading, some build-up, and other factors increasing system resistance are considered for fan and motor selection. Keep in mind that as the system resistance goes up in a dust collection operation, the fan will not generate as much air flow. As a result, particle settlement (called saltation when in a horizontal pipe or duct section) leading to build-up and/or complete plugging may occur. Many fans will lose over 30 per cent of their air flow when the system resistance doubles. Usually with a dust collection system a variable speed motor is not necessary, though it can be ‘nice to have’.
• Tip #4 – Avoid plugging. Dust collection system plugging can manifest in many forms, such as in the duct, hopper, or filter. The root causes can be plentiful: poor duct layout (too many bends and/or multiple bends in a row), over-feeding the line, air leaks, build-up in piping, and hopper/receiver plugging. There are many instances where the conveying velocity was correctly selected and the fan and motor operated as required, but the dust did not convey effectively through the duct.
• Tip #5 – Don’t ignore the separator. Separator performance can make or break the dust collection system. A properly performing separator will efficiently filter the particles from the air stream, clean itself, discharge the solids into a hopper, and allow proper air flow through its media, thereby maintaining conveying velocities in the system and stable fan operation. Poor separator performance can allow particle bypass through the filter, clogging and plugging, and substantially reduced air flow rendering dust pickup and conveying moot.
Collecting the dust generated during handling can be expensive given the potential large capital cost of dust collection equipment (dust collectors, filters, or special dust collecting housings). Furthermore, the collected dust will have to be either disposed of (incurring additional cost) or installation of a recycle system to feed material back into the process.
DUST SUPPRESSION
An alternate approach to reduce dust is by adding moisture or chemical dust suppressants (salts, emulsions, surfactants, polymers, lime sprays, foams, see Figure 5) to the bulk solid before or during handling [4,5,6]. Although this may be a helpful method for minimizing dust, adding moisture, or suppression agents often result in a more cohesive material causing flow issues such as unreliable discharge from storage bins or build-up/plugging of transfer chutes.
While the use of dust collection systems and suppression may be effective methods for minimizing dusting, modifying, or replacing improperly designed equipment (such as a transfer chute) should be considered as it may result in significant reductions in dust generation.
DESIGN FOR DUST MITIGATION IN TRANSFERS
Dust is often created in a transfer chute when the flowing material entrains air. A beneficial tactic is to design transfer chutes to prevent dust from becoming airborne in the first place. Myriad techniques could be used to accomplish this task, including:
• Keep the material in contact with the chute surface.
• Concentrate the material stream.
• Keep impact angles small.
• Keep the velocity through the chute as near constant as possible.
• Make sure that the particles leaving the chute are traveling in the direction of and close to, or greater than, the velocity of the receiving belt.
By following these techniques, the amount of dust generated at a transfer point can be reduced by orders of magnitude, if not eliminated completely. [7]
CAUTION – DUST EXPLOSION HAZARD
In a recent survey of nearly 250 engineers working in industrial plants that handle, store and process bulk solids, only 15 per cent indicated that they knew the explosivity potential of their materials. Nearly 70 per cent of all dusts are flammable, meaning they can explode as a dust cloud. Consider this in the context that around 50 per cent of dust explosions occur in dust collectors. [8] Many dusts can burn rapidly either in a flash fire or an explosion scenario. Though most people are aware of the hazards of flammable gases and liquids, some may not know the hazards of combustible dusts handled in storage or dust collection equipment [9]. A dust explosion requires five key ingredients (see Figure 6):
• combustible dust (e.g., sugar, plastic, wood, metals, and most carbon-containing dusts)
• oxidant (oxygen is present in air in most process areas)
• ignition source (static discharge, hot surfaces, sparks)
• dispersion (dust can be readily emitted from numerous sources)
• confinement (dust collector, silo, dryer, mill/grinder)
Even if confinement is eliminated from the explosion pentagon, a dangerous flash fire deflagration event can still occur causing damage to property, injuries, and possibly loss of life. This further highlights the need for a proper dust collection system. Without it, dust can accumulate on horizontal surfaces creating opportunity for a hazardous flash fire event. Excellent guidance for preventing and protecting against combustible dust hazards are presented in several NFPA standards [10]. (Author’s note: Do not ignore protecting the dust collector as it provides the perfect conditions for a deflagration. There are many methods to protect the equipment and process through explosion containment, isolation, suppression, and venting. NFPA 69 provides excellent guidance for each of these approaches).
Dust control is one of the most pervasive issues relating to materials handling especially at mine sites and processing plants. With operations handling massive quantities of fine bulk materials every day the dust generated can become a serious health and safety hazard. Applying proper design techniques can dramatically mitigate dust generation and the associated risks.
Do you have a bulk solids handling question? Jenike & Johanson has developed the science of bulk solids flow and specialises in applying it to solving the most challenging bulk solids handling problems. So why not put them to the test with your question? The harder, the better.
References
1 Wikipedia: https://en.wikipedia.org/wiki/Dust_collector
2 Maynard, Eric, “Six Key Considerations for Proper Dust Collection System Design”, Powder & Bulk Engineering magazine, Volume 32, No. 10, October 2018, pp. 22-33
3 Dust Control Handbook for Industrial Minerals Mining and Processing; RI 9689/Report of Investigations, Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, Spokane, Washington; 2012.
4 Bader, Charles D.: Controlling Dust, Erosion Control, July/August 1997.
5 Zador, Andrew: Methods to Prevent Environmental Pollution at Large Bulk Material Handling Terminals, Bulk Solids Handling, September 1993.
6 Baeyens, J., Smolders, K., Dumont, Ph.: Stockpile Dust Emission and Suppression, Bulk Solids Handling, May/June 2001.
7 Petro, G.J.: Chute Design for Effective Pet Cokes Handling, Solids Fuel Handling, 1996.
8 Eckhoff, R.K., “Dust Explosions in the Process Industries,” 2nd edition, Elsevier, 1997.
9 Maynard, E., “What is a Dust Hazards Analysis (DHA) and why do I have to worry about it?” Australian Bulk Handling Review, May/June 2018.
10 www.nfpa.org/652, NFPA 652, 2016 edition: Standard on the fundamentals of combustible dust.